Some electronic musical instruments produce musical tones by reading the data about waveshapes of musical tones from a waveshape memory. In a known instrument of this type, as exemplified in U.S. Pat. No. 4,383,462 issued on May 17, 1983 and assigned to the same assignee as the present application, the data about the whole waveshape of a musical tone beginning with its attack portion and ending with its decay portion is previously stored in a waveshape memory, and when the musical tone is to be produced, the data is read from the memory. This prior art instrument is able to produce musical tones closely resembling those of acoustic, non-electronic musical instruments, and it is especially adapted to obtain percussive sounds, as produced by percussions, whose timbres and pitches change with time. Unfortunately, it requires an exorbitantly large capacity of memory to store data about waveforms.